Field of the Invention
The present invention concerns a method to determine a B1 phase map for at least two excitation modes of a radio-frequency (RF) coil arrangement of a magnetic resonance (MR) apparatus that is controlled by multiple independent transmission channels, the B1 phase map describing with spatial resolution, the phase of the radio-frequency field that is generated in, a particular excitation mode, with respect to a common reference phase map. The invention also concerns a magnetic resonance apparatus designed to implement such a method.
Description of the Prior Art
Magnetic resonance imaging is a widely known medical imaging modality. A subject to be examined is introduced into a basic magnetic field with a relatively high field strength (known as the B0 field). In order to acquire magnetic resonance data, for example in a slice of the subject, nuclear spins of this slice are excited and the decay of this excitation produces a signal. Gradient fields are generated by a gradient coil arrangement while radio-frequency excitation pulses (also frequently designated as radio-frequency pulses) are emitted via a radio-frequency coil arrangement. The entirety of the radio-frequency pulses (excitation) generates a radio-frequency field that is typically designated as a B1 field, and the spins of resonant excited nuclei are deflected (flipped), with spatial resolution due to the gradients by an amount known as a flip angle, relative to the magnetic field lines of the basic magnetic field. The excited spins of the nuclei then radiate radio-frequency signals that are acquired and processed further by suitable reception antennas (such as by the same radio-frequency coil arrangement used for excitation) in order to be able to reconstruct magnetic resonance image data.
Conventional radio-frequency coil arrangements are operated in a manner known as “homogeneous mode”, for example in a CP mode (circularly polarized mode), wherein a single radio-frequency pulse is emitted with a defined, fixed phase and amplitude to all components of the transmission coil, for example all transmission rods of a birdcage antenna. To increase the flexibility and to achieve new degrees of freedom to improve the imaging, it has been proposed to operate in a manner known as a parallel transmission (pTX), in which multiple transmission channels of a radio-frequency coil arrangement are individually charged (supplied) with individual pulses that can deviate from one another. This entirety of the individual pulses (which, for example, can each be described by the parameters of phase and amplitude) is then defined as a whole in a control sequence that is defined by a parameter set. Such a multi-channel pulse (excitation) that is composed of individual pulses for the different transmission channels is often designated as a “pTX pulse” (for “parallel transmission”). In addition to the generation of spatially selective excitations, field in homogeneities can also be compensated (for example within the scope of “RF shimming”).
In order to determine control parameter sets of a control sequence, it is necessary to know the background (thus the B0 field), and to know the effects of the individual transmission channels in the imaging region (in particular the homogeneity volume) of the MR apparatus.
For measurement of the basic magnetic field (B0 field)—designated as a B0 mapping—first magnetic resonance data are typically acquired (preferably via gradient echo imaging) at two different echo times. The phase difference (phase change) of the magnetic resonance data acquired at the different echo times (which can be determined by subtraction of the phases of two magnetic resonance images of the first magnetic resonance data that are acquired at different echo times) is proportional to a deviation of the local B0 field from the nominal basic magnetic field strength, and to the dephasing time (thus the difference of the two echo times). The field deviation is thereby specifically described by a deviation of the Larmor frequency from a nominal Larmor frequency of the magnetic resonance device (a value describing this deviation is designated as a Larmor frequency value in the following).
The phase generated by deviations in the homogeneity of the B0 field thus develops over time, so the effect of the Nyquist phase wrapping must be taken into account because the proportionality of the phase difference of magnetic resonance data acquired at different times to the deviation from the nominal Larmor frequency, and to the difference of the echo times, applies only as long as the phase difference (limited to 2π) corresponds to the actual phase evolution. However, the phases can be further developed by multiples of 2π depending on the dynamic range of the B0 distribution. This leads to ambiguities and errors in the calculation of the B0 maps. Incorrect associations in the phase evolution manifest themselves as non-physical spatial discontinuities due to the 2π jumps in the phase difference images. This thus means that an extremely fast development of the B0 phase also occurs if the deviation of the local Larmor frequency from the nominal Larmor frequency is high, such that the phase will go beyond 2π when the echo time (here the difference of the two echo times) is not short enough, such that the described ambiguity occurs.
The selection of extremely short dephasing times is often not possible due to the sequences that are used, because smaller deviations from the nominal Larmor frequency can no longer be measured with sufficient precision given an extremely short echo time difference.
A few approaches are known in the prior art to solve the ambiguity problem in the association of the measured phase change. It is thus possible to choose the dephasing time (thus the difference of the echo times) to be so short that the phases do not develop by more than 2π at any location during them. However, since the dynamic range of the B0 field distribution is not known before the measurement, the dephasing time must be chosen to be so short that the sensitivity of the acquisition method is not sufficient, and this procedure is consequently not used (as already explained).
Therefore, it has been proposed to detect and correct phase jumps in the B0 maps in post-processing, under the assumption that the B0 field is spatially continuous. Algorithms that are used for this purpose are known as phase unwrapping algorithms. However, the reliability of such algorithms is often questionable. The primary difficulty is that the entire volume can be composed of non-contiguous partial regions, such that individual partial regions of the B0 maps are separated by voxels that include only noise and are very low in signal. The phase in these voxels can thus not be determined, or can only be determined unreliably.
It has also been proposed to iteratively acquire first magnetic resonance data with increasing dephasing time, consequently increasing difference between the echo times. The shortest dephasing time is thereby selected so that no spatial phase jumps occur. Whether a phase jump will occur given longer dephasing time is estimated from the acquisitions with shorter dephasing times. If this is the case, this is taken into account in the evaluation (reconstruction) of the first magnetic resonance data with longer dephasing time. The phase ambiguity is therefore dispelled, and long dephasing times are enabled for a high sensitivity.
A further alternative procedure is to minimize the phase gradients between adjacent voxels in the B0 maps. In this approach, the B0 maps do not necessarily need to be corrected for phase jumps. However, there is a risk that a calculated B0 shim is optimized for false B0 offsets in different spatial areas. Moreover, no frequency (zeroth order shim) can be calculated from the differential method.
Mapping processes are also known for B1 fields and are designated as “B1 mapping”. In general, B1 field maps are acquired for each transmission channel, which means that the B1 field maps show how strong the B1 field is at a specific location in the imaging region given a specific excitation (for example a uniform excitation and/or given a defined transmitter voltage), which means that a complex B1 value (consequently a B1 amplitude and a B1 phase, which can also be differentiated in a B1 amplitude map and a B1 phase map) is associated with each voxel (image point). Typical measurements for a number of excitation modes are thereby conducted. The excitation mode does not necessarily need to be the operation of only one channel; rather, combinations are also possible from which the effect of individual transmission channels can then be concluded.
In order to determine the amplitude of the B1 field, for example, it is known to measure the flip angle that a radio-frequency pulse causes, as described in DE 10 2005 049 229 B3, for example. An excitation mode thereby results in a constant phase shift (consequently a constant B1 field). The B0 phase, that is continuously varying over time (as described), is naturally also acquired in the phase measurement. Therefore, for B1 mapping it is known to use basically the same echo time for the different excitation modes, such that the effect of the B0 field on the phases is kept constant.
If the magnetic resonance data acquired in the B1 mapping are designated as second magnetic resonance data, in the prior art a raw phase map (obtained from the second magnetic resonance data of an excitation mode) is often used as a correction, which means that the correction raw phase map is subtracted from all other raw phase maps so that the consistent effect of the B0 field on the phase drops away, and consequently the phases of the excitation mode used for correction serve as a reference. This means that all other B1 phase maps are defined relative to the B1 phase map used as a correction, which is unproblematic, because it ultimately depends only on the relative phases of the different transmission channels in any event.
This procedure is problematic at higher B0 fields (higher than 1.5 T, for example), because then it is difficult to ensure that the reference excitation mode (or a combination of multiple reference excitation modes) will produce sufficient excitations over the entire imaging region (in particular the subject to be acquired). A defined excitation mode will typically exhibit regions of low excitation sensitivity (transmission sensitivity), such that a smaller flip angle is present in those regions, while in other regions an “over-flipping” can occur so that a low signal-to-noise ratio (SNR) and a poorly defined reference phase are present. Because all other B1 phase maps are determined relative to this reference B1 phase map (in the described correction process), the inaccuracies are ultimately adopted in all other B1 phase maps.
Although it is conceivable in principle to define the B1 field maps without reference to other B1 field maps, the effects of the local Larmor frequency are then still present for the given echo time. As a result, although the B1 field maps determined in such a manner are still correct relative to all other B1 field maps determined in such a manner, they exhibit regions with phase wrapping. This can entail difficulties for the design of the aforementioned control sequences. Because the measurements (data acquisitions) to determine the second magnetic resonance data are in addition to other data acquisitions, phase drift and other time-dependent effects can lead to a displacement (shift) of the borders of the regions of the phase wrapping, such that additional difficulties can occur in the post-processing and in drawing conclusions from the B1 field maps.